Sounds can knock drones out of the sky

Knocking a drone out of the sky is sometimes possible using an invisible weapon -- sound.

The main problem is aiming and tracking. "We are not Lockheed Martin or Northrop Grumman," Kim said. "We cannot make those weapons in our lab."

Still, they tried some small-scale efforts. In one experiment, Kim's team embedded an audio module into a police shield. The problem with the shield is that it is difficult to continually track a drone in order to disrupt it. Even if the attack is initially successful, the drone starts to move so erratically that it become hard to track.

"Directionality matters a lot," he said. "It was pretty difficult."

In another attempt, they built a kind of sonic wall, comprised of an archway containing speakers. The problem with the archway is that the drone passes too quickly through the ideal attack zone.

But there are a variety of sound-related offensive and defensive devices already on the market. For example, the LRAD Corporation makes the 450XL, which it terms an "acoustic hailing device." It can be mounted on a vehicle or a tripod and can project a voice message up to 1,700 meters.

Japanese fisherman have used sound devices against environmentalists trying to disrupt their whaling operations, focusing intense, loud sounds against pursuers.

Combined with a tracking radar, those type of devices could make such attacks on drones more viable. So how to defend against a sound attack? The key is insulating the gyroscopes from interference, using shielding or foam.

"With a good casing, you can prevent most of our attacks," Kim said.

The research paper was also co-authored by Yunmok Son, Hocheol Shin, Dongkwan Kim, Youngseok Park, Juhwan Noh, Kibum Choi and Jungwoo Choi, all of KAIST.